Involvement of indole-3-acetic acid produced by the growth-promoting bacterium Azospirillum spp. in promoting growth of Chlorella vulgaris

Involvement of indole-3-acetic acid (IAA), produced by the microalgae-growth-promoting bacteria Azospirillum brasilens and A. lipoferum, in promoting growth of the microalga Chlorella vulgaris Beij. was studied. Four wildtype strains of Azospirillum and their IAA-deficient mutants were co-immobilized with C. vulgaris in alginate beads. Cultures were grown in synthetic growth medium supplemented with tryptophan. Growth promotion of microalgae and production of exogenous IAA by Azospirillum spp. were monitored. All wildtype Azospirillum spp. produced significant but varying amounts of IAA, while their mutant forms produced significantly less. The results demonstrated a significant growth promotion in Chlorella cultures when immobilized with the four wildtype strains of Azospirillum, while very low or no enhanced growth was induced by the four IAA-deficient mutants, compared to when C. vulgaris is immobilized alone. A complementation experiment, where an IAA-attenuated mutant (A. brasilense SpM7918) was supplemented with IAA produced by its parental wildtype strain (A. brasilense Sp6), restored growth promotion in the microalgae-mutant culture.